collapse cross-account dupes, Lato UI font, mail-list selection & density tuning

Backend:
- Collapse identical cross-account deliveries in a thread: new messages.delivery_id
  (migration 0011) preserves the real Message-ID even when message_id_hdr is
  synthesized (§7.1); get_thread groups by it and reports also_in_account_ids; the
  list count counts distinct deliveries. Backfill script for existing rows.

Frontend:
- Bundle Lato (SIL OFL) as the UI font; unread mail-list rows use Black weight.
- Mail-list selection: hover-avatar checkbox, tri-state "select all", bulk
  Archive/Pin/Delete (Restore/Delete in Archive/Trash/Spam) with a count in the
  reading pane; unread blue dot; "also delivered to" note in the thread.
- Spacing/size tuning: sender-column width, icon gaps, avatar size, bar heights,
  and a round of -1px font sizes across the sidebar, mail list, and reading pane.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U72FZiF1W1pb8XxyWBTREA

backend/app/models.py

@@ -157,6 +157,10 @@ class Message(Base):
         UUID(as_uuid=True), ForeignKey("threads.id")
     )
     message_id_hdr: Mapped[str | None] = mapped_column(Text)
+    # The real header Message-ID as delivered (kept even when message_id_hdr is
+    # synthesized for a cross-account duplicate, §7.1). Identifies one delivery, so
+    # the same email fanned out to several accounts collapses to one row in a thread.
+    delivery_id: Mapped[str | None] = mapped_column(Text)
     in_reply_to: Mapped[str | None] = mapped_column(Text)
     references_hdr: Mapped[list[str]] = mapped_column(
         ARRAY(Text), nullable=False, server_default=text("'{}'")